Conventionally, some diesel engines have a selective reduction catalyst incorporated in an exhaust pipe for flow of exhaust gas, said catalyst having a property of selectively reacting NOx with a reducing agent even in the presence of oxygen. A required amount of reducing agent is added upstream of the selective reduction catalyst and is reacted on the catalyst with NOx (nitrogen oxides) in the exhaust gas to thereby reduce a concentration of NOx discharged.
Effectiveness of ammonia (NH3) used as a reducing agent for reduction and depuration of NOx is well known in a field of a plant or the like with industrial flue gas denitration. However, in a field of automobile where safety is hard to assure as to running with ammonia itself being loaded, proposed is use of nontoxic urea water as a reducing agent.
More specifically, the urea water, which is added into the exhaust gas upstream of the selective reduction catalyst, is hydrolyzed in the exhaust gas into ammonia and carbon dioxide gas according to a following equation to thereby satisfactorily reduce and depurate NOx in the exhaust gas with the ammonia on the catalyst.(NH2)2CO+H2O→2NH3+CO2  [Equation 1]
Shown in FIG. 1 is an example of conventional art for addition of urea water into exhaust gas upstream of a selective reduction catalyst. In the example illustrated, an outer periphery of an exhaust pipe 1 is formed with a bulge portion 2 protruding obliquely upstream. Attached upstream of the bulge portion 2 is an injector 3 directed obliquely downstream to approach an inside of the exhaust pipe 1 so as to add urea water 5 while the injector 3 is protected from being directly exposed to the flow of the hot exhaust gas 4.
Arranged in the exhaust pipe 1 immediately after the bulge portion 2 is a mixer 6 constituted by thin grate plates with planar surfaces in a direction of the flow of the exhaust gas 4 such that the urea water 5, which is sprayed from the injector 3 to the mixer 6 having been exposed to and elevated in temperature by the flow of the exhaust gas 4, is atomized and heated for facilitated conversion into ammonia.
There is, for example, the following Patent Literature 1 as conventional art document concerning this kind of mixing structure.